This invention relates generally to antenna systems, and more particularly to the use of shaped serrated edges reflector surfaces to optimize electromagnetic scattering in off-axis directions and to enhance electromagnetic scattering in the on-axis direction. The shaped, serrated edges can be used with center-fed or offset-fed parabolic and quasi-parabolic dish reflectors and cylindrical, spherical arid planar reflectors used for antennas or with compact ranges.
The present abrupt or irregular edge shapes cause high scattering of electromagnetic energy in off-axis regions resulting in high sidelobe levels for antenna and reflector application, or large amplitude ripples in the quiet zone for compact range applications.
The field reflected from a parabolic main reflector surface stops abruptly at the surface termination. The diffracted field emanating from the edges of the reflector contaminates the plane wave in the quiet zone. The compact range has been used for many years to measure directive patterns of microwave antennas. Typically, the source antenna is used as an offset feed that illuminates a paraboloidal reflector which converts the impinging spherical wave into a plane wave. Compact ranges can also be used for scattering measurements but require a lower level of stray signals coming from the reflector edges.
To improve the field quality in the quiet zone, one approach has been to use a rolled edge structure with the basic parabolic reflector. W. D. Burnside, M. C. Gilreath, and B. Kent, "A Rolled Edge Modification of Compact Range Reflector," Antenna Measurement Techniques Association, 1984 Conference, Oct. 2-4, 1984. Although this approach improves system performance, it requires excessively large and expensive edge structures. An improvement on this approach has been to use a blended, rolled surface which provides a smooth transition between the parabolic surface and the rolled edge. C. W. I. Pistorius and W. D. Burnside, "A Concave Edged Reflector With Blended Rolled Surface Terminations For Compact Range Applications," Antenna Measurement Techniques Association Annual Meeting and Symposium Sep. 23-25, 1986.
The most cost-effective approach for a given quiet zone size is to use edge serrations since they require only edge cutting and not a different surface shaping. Edge serrations provide a transition of reflected field strength from the high relative levels in the center area of the reflector, corresponding to the test zone area, to zero at the outer periphery of the reflector. However, edge serrations are typically designed from a quasi-Geometric Theory of Diffraction (GTD) point-of-view and not from the field transition point-of-view. Using this new field transition point-of-view can produce better quiet zone performance than previous edge serration shapes.
Also in the prior art is the patent to Holtum, U.S. Pat. No. 3,599,219. Holtum discloses providing an edge configuration to a circular parabolic dish antenna to reduce backlobe radiation. By providing an edge configuration, successive portions of the edge are at differing distances from the feed resulting in variation of the phase of radiation diffracted from successive portions of the edge. Holtum discloses that it is desirable that the conducting extensions depart outwardly from the parabolic shape of the main body of the reflector, regardless of whether it is constructed as an integral extension of the reflector surface or as a separate addition. Furthermore, Holtum teaches the use of a circumscribed polygon structural shape for addition to the circular structure of a dish antenna to achieve a continuously varying radius. He states that to achieve his goal of dispersion of the axial backlobe into numerous sidelobes does not require intricate and precise design of the edge of the diffracting structure.
The Russian patent to Glazman, SU 1137-547-A, discloses the addition of one or two flat diffraction screens to cause a dephasing along the reflector edges producing a suppression in the diffraction of the radiation field by at least 6 dB.
The Japanese patent to Hirukoi discloses the use of a shielding plate around the reflector to limit propagation from the reflector to two planes. The shielding plate is provided in the plane orthogonal to the direction of the incident wave. The shape of the edges of the shielding plate is formed by combining lines parallel to two specific directions in the plane.